Abstract

Phenotypic and genotypic characteristics of cowpea rhizobia indigenous to soils of Ethiopia are unknown. Forty indigenous cowpea rhizobial isolates were collected from cowpea-growing areas of the country and were characterized for their growth and genetic properties. Based on their cultural characteristics, the isolates were categorized into fast (FG), slow (SG), and extraslow-growings (ESG). The FG, SG, and ESG isolates had mean generation time (h)/colony diameter (mm)/date of turbidity formation (d) in the range of 2.5-7.5/2-4/2-3, 7.5-30/0.5-3.5/3-5, and 30-50/0.5-1/5-7, respectively. Thirty two and sixteen percentages of the isolates were FG and ESG, respectively. Most of the isolates (87%) could grow on culture medium of pH 4.5, but were intolerant of pH 8. The intrinsic antibiotics resistance (IAR) pattern was FG>SG>ESG for ampicillin, ciprofloxacin, chloramphenicol, erythromycin, neomycin sulfate and penicillin, whereas the pattern was FG<SG<ESG for gentamycin sulfate salt and nalidixic acid. The C-source utilization pattern was FG>SG>ESG for dextrin, dextrose, glucose, starch and sucrose whereas it was FG<SG<ESG for arabinose and galactose. Only FG isolates grew on culture medium containing methionine as sole N source. Cluster analysis of the isolates based on their phenotypic and genotypic characteristics matched with their growth categories in which 65% of the isolates that had mean generation time of >7.5 h were grouped together at 70% of similarity. Partial sequence analysis of 16S rRNA gene showed the existence of isolates most similar to rhizobial species of Bradyrhizobium species, Bradyrhizobium japonicum, Bradyrhizobium elkanii, Rhizobium rubi, and Mesorhizobium species. In general, cowpea rhizobial isolates from soils of Ethiopia in this study were mainly SG and sensitive to stress in vitro conditions, but versatile in utilization of varieties of C and N substrates. Such studies are important in Ethiopia to identify rhizobial isolates that could be amendable for use as inoculants to improve cowpea production.

Key words: Phylogeny, diversity, isolates, growth categories, cluster analysis.